The present invention relates to semiconductor integrated circuits, and more particularly to standard cell libraries and integrated circuits including cells from standard cell libraries.
As sizes of integrated circuits (ICs) increase, semi-custom design schemes are generally adopted, because it may be difficult, expensive and/or time consuming to design all circuits manually. In semi-custom design schemes, a plurality of standard and basic cells are prepared in advance, and a desired circuit is developed by designing logic cells automatically using the standard cells by a computer. The standard cells may be stored, for example, in a standard cell library that is stored in a computer readable database. Accordingly, a standard cell library is a database of standard cell designs, or computer readable layout instructions, that define cell layouts that can be used by a designer to design a logic circuit. For example, a standard cell scheme may be used as one of the semi-custom design schemes. In the standard cell scheme, relatively complex logic circuits may be designed by combining the basic cells, and stored as standard cells in a database of the computer. In case of designing an IC, a desired IC may be implemented by combining the standard cells stored in the database. A plurality of cells having a same cell height may be arranged in a plurality of rows.
As the integration level of the IC increases, there is a need for reducing cell size. Generally, if the size of a transistor in each cell is reduced, it is possible to reduce the cell size. However, when sizes of all transistors of the cell are uniformly reduced, it may become more difficult to implement a desired function. In addition, when sizes of all transistors of the cell are uniformly reduced, a complexity of the lowest metal layer may increase, because a conventional standard cell usually uses only the lowest metal layer, and thus the effective cell size may increase rather than decrease.